Various powder mixtures from the starting powders of Ti/Si/C, Ti/SiC/C, Ti/Si/TiC, Ti/SiC/TiC and Ti/TiSi 2 /TiC were used for the synthesis of ternary compound titanium silicon carbide (Ti 3 SiC 2 ) by using a pulse discharge sintering (PDS) process. The Ti/Si/TiC powder was found to be the best among the five powder mixtures for Ti 3 SiC 2 synthesis. Phase purity of Ti 3 SiC 2 can be improved to %99 mass% at the sintering temperature of 1300 C for 15 min. The relative density of all the synthesized samples is higher than 98-99% at the sintering temperature above 1275 C. The nearly single phase Ti 3 SiC 2 was found to show plastic deformation at room temperature and good machinability. Both electrical and thermal conductivity were found to be greater than two times of the values of a control pure Ti sample fabricated by the same sintering process. The thermopower of the synthesized Ti 3 SiC 2 was measured to be nearly zero in the testing temperature range, much lower than some common low thermopower substances such as gold or carbon. Their mechanical properties at ambient and elevated temperatures were also examined. The ternary compound Ti 3 SiC 2 is referred to as a ''metallic ceramic'' according to its physical and mechanical behavior representing both metals and ceramics.

Recently developed advanced high-strength materials like metallic glasses, nanocrystalline metallic materials, and advanced ceramics usually fracture in a catastrophic brittle manner, which makes it quite essential to find a reasonable fracture criterion to predict their brittle failure behaviors. Based on the analysis of substantial experimental observations of fracture behaviors of metallic glasses and other high-strength materials, here we developed a new fracture criterion and proved it effective in predicting the critical fracture conditions under complex stress states. The new criterion is not only a unified one which unifies the three classical failure criteria, i.e., the maximum normal stress criterion, the Tresca criterion and the Mohr-Coulomb criterion, but also a universal criterion which has the ability to describe the fracture mechanisms of a variety of different high-strength materials under various external loading conditions.

No abstract

The microstructures of pure Cu processed by equal channel angular pressing (ECAP) from 4 to 24 passes were investigated. It was found that the microstructures of Cu samples with a small number of ECAP passes (4-8) were not inhomogeneous and the fraction of high-angle grain boundary (HAGB) was low (25~43%). While for the samples with many number of ECAP passes (12-24), the grains became more equiaxed-like and the GB misorientations exhibited double-peak distribution with high fraction (51~64%) of HAGB. It was dislocation cells formed in large grains of the few-pass samples, but subgrains in the many-pass samples. These characterizations suggested that ultrafine-grained (UFG) microstructures in the few-pass samples were not fully accomplished, while it was obtained after many passes (>12). It is believed that dynamic recovery during processing for many passes was attributed to the formation of UFG microstructures.